The present invention relates generally to lighting applications, and more particularly to a light source comprising an organic light-emitting device combined with a layer of photoluminescent material, which may have a relatively large area.
There are many examples of lighting devices that comprise inorganic light-emitting diodes (LEDs) or organic light-emitting devices (OLEDs). One example is the commercially available blue inorganic gallium nitride (GaN) LEDs that are coated with phosphor particles to produce white light. These LEDs are essentially point sources of light rather than extended sources of light.
An example of an OLED is set forth in U.S. Pat. No. 5,294,870, which describes an organic electroluminescent multicolor display device comprising an organic electroluminescent source emitting blue light with green- and red-emitting fluorescent materials applied to different subpixel areas. This device emits different colors from the different subpixel areas by color shifting with the green- and red-emitting fluorescent materials.
Another example of an OLED is described in Junji Kido et al., “Multilayer White Light-Emitting Organic Electroluminescent Device”, 267 Science 1332-1334 (1995). This device includes three emitter layers with different carrier transport properties, each emitting blue, green, or red light, which layers are used to generate white light. In this device, however, the layers emitting the different colors typically degrade over time at different rates. Consequently, the color of the device is likely to change over time. In addition, the uniformity of the light output over the emitting area of the device may be less than desirable.
However, prior-art OELDs often suffer from low efficiency, a cause of which is an incomplete capture of charges within the organic light-emitting layer to produce useful high-energy species.
Therefore, there still is a continued need for efficient OLEDs. In addition, it is very desirable to provide more efficient OLEDs that may be used as light sources.